Acoustic direction finding is the task of finding the direction of a sound source given measurements of the sound field. The sound field can be described using physical quantities like sound pressure and particle velocity. A typical approach in artificial systems is to utilize two (or more) microphones and evaluate a difference of arrival times or pressure, allowing mathematical estimation of the direction of the sound source. However, the accuracy of these systems is fundamentally limited by the physical size of the array. Generally, if the array is too small, then the microphones are spaced so closely that interaural time differences approach zero, making it extremely difficult to estimate the orientation. As a result, effective microphone arrays may become cumbersome and impractical for use on smaller mobile platforms, or as a personal device.
Animals similarly use their hearing to identify the direction of an auditory stimulus when both of the ears are excited by a sound wave, based on differences in arrival times and in the intensity of the sound between the nearest and the furthest ear. In the case of large animals, differences in intensity and the arrival time are relatively large and easily detected. However, smaller animals, experience small interaural differences. As a result, many small animals have developed mechanisms for effectively increasing these differences before the sound stimulus reaches the auditory cells.
This behavior as served as the inspiration for development of small-dimensioned, microelectromechanical direction finding sensors. See e.g., U.S. Pat. No. 8,467,548 to Karunasiri et al., issued Jun. 18, 2013. This particular sensor provides localization of sound sources using sensors much smaller than the wavelength detected, by utilizing bending mode stimulated by the effect of incident sound pressure on the sensors wings. However, the symmetric response of the sensor makes the determination of bearing ambiguous.
Similar bearing ambiguity in sound direction systems is not a new issue, and various solutions are typically utilized. The problem of bearing ambiguity can be resolved by altering the position of a sensor relative to a sound location, for example, maneuvering a ship to provide a different geographic location of reception. These two techniques can work well as long as the target has not moved significantly before and after re-location, but they can lead to inaccurate conclusions if the sound source is moving at a relatively high speed, and additionally incurs an obvious delay in location while the sensor is relocated. Such a delay may be unacceptable or highly impractical in certain situations, such as a first responder or soldier attempting to locate a source of apparent gunfire.
It would be advantageous to provide an acoustic direction finding system which could be easily deployed on smaller mobile platforms or as a personal device, and which allowed relatively instantaneous direction finding without delays associated with necessary relocation of the sound sensor. Such as system would be highly useful for first responders, soldiers, and others, as well as for smaller, mobile robotic or other units which might seek to employ such direction finding for navigational purposes.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.